Jetbike
Jetbikes are freaking sweet rides that any fan of sci-fi will know and imagine owning one day. They fly so no fear of getting a flat tire or getting bogged down need trouble the enthusiastic soon to be dead nujack pilot (what with probably hitting an unseen protruding object below that catches and flips you onto your poor vulnerable skull or running right into an object a la Imperial Bike Scouts in Return of the Jedi.) They often have sweet weaponry that you can fire as you streak towards your enemy, meaning that between your firepower and speed you will often outdo them easily. And like the Eldar of Saim-Hann and the Ork Speed Freakz, you can get that rush from going really, REALLY fast.
As with everything cool and deciding humanity needs to be fucked from every angle possible, GW has decided the Imperium doesn't need Jetbikes and has thus made it a lost technology to the Imperium, dumping your proud marines back onto tired (pun intended) old bikes.
Boo!
To rub grimsalt in the wound, GW has done two things. First of all the pesky Eldar and the wimpy Tau both have access to Jetbike/Jetbike-like rides, so they can fly around blowing raspberries at you. (I guess Marines do get land speeders but that is like a car to the motorbike that is the Jetbikes. Instantly not as trendy). So if the enemies of the Imperium having it wasn't even, there is the second thing. During the Great Crusade the Imperium DID have Jetbikes. Only in small numbers but growing and the Legiones Astartes used them as dedicated fast response units and to pick up chicks (check out this motor baby!).
But the Imperium is ridiculous at keeping back-up copies of all it's technical data and they just....forget or lost the means to make Jetbikes. Such a gross example of fail that has continued to haunt the Imperium and Imperial players to this day. (Heck I wonder if the Imperium is even aware now they used to have Jetbikes.....hmmm).
The only remaining known one is used by the Master of the Ravenwing of the Dark Angels Chapter, Sammael, who uses it to hunt Fallen Angels HERETICS FROM THE NINE TRAITOR LEGIONS. Like the rest of the Dark Angels, he is a dick who doesn't share the fact outside of the chapter he has a working Jetbike. Of course if you thought that would mean the Imperium could, you know, start making Jetbikes again for it's armed forces, think again. The Adeptus Mechanicus is so anal about anyone touching technology they would probably hide it in a museum somewhere to gawk at on some stand.
Then again, it wouldn't be out of character for the Dark Angels to have secretly collected a bunch of jetbikes (or even an STC) and lied about it to the rest of the Imperium, as Sammael's jetbike has been working surprisingly well for many centuries despite dragging into a warzone after another, not to mention the numerous times it's been seemingly destroyed, only to reappear without a scratch on it...
Basically the Imperium has no Jetbikes left because everyone who might have access to one is a greedy superstitious twit who would rather hoard their goodies then share. And who are the victims? WE ARE because we don't get to have Jetbikes in our armies.....(fail)(but with the release of Horus Heresy models, our armies can have them now, YAY!(they even have rules DOUBLE YAY!!))